I liked the opening until I realized you were not talking about Zombie Shooter. Really...Very good review. Horde, swarm and infestation do not quite do justice to the shear number of zombies in this game. (I watched a youtube video on this game and wow.)

You say "pay experience points" which leads me to believe there is some sort of hybrid RPG element to this game. Do you actually gain levels and hit points as you progress?